The present invention relates to an ink jet writing apparatus, and in particular to an ink jet writing apparatus having means for preventing the clogging of ink when the writing head is not in operation.
Ink jet printing has been known in the art as exemplified by the system shown and described in U.S. Pat. No. 4,106,032. In the system described in this patent, a jet of writing fluid or ink is caused to issue from a nozzle in the form of a succession of tiny individual droplets of a varying size depending on the instantaneous value of an input signal which is to be recorded. The nozzle is moved across the surface of a recording medium so that the ink droplets are ejected to desired print positions.
In a writing system of the type described above, nozzle clogging has presented a well known problem. One of the reasons of clogging is due to the ink drying in the nozzle over a standstill period. The nozzle has an inside diameter in the order of 40 micrometers and ink allowed to stay therein tends to clog the opening when the ink has dried. Prior attempts have been made to solve this problem. For example, United States patent application No. 20,977 filed Mar. 16, 1979, now U.S. Pat. No. 4,223,324, discloses a system in which a solenoid-operated rubber-coated pad is brought up to close the nozzle opening to prevent the ink from drying when the system is not in use. In the disclosed system, moisture laden air is forced through a space between the pad and the nozzle opening when the system is at standstill to moisten the nozzle opening.
However, this system is still not satisfactory for a number of reasons. Firstly, since the contacting surface of the rubber-coated pad and the front face of the writing head on which the nozzle opening is provided are not strictly parallel to each other due to manufacturing errors, it is difficult to provide a fluid-tight chamber that encloses the nozzle opening, so that moisture laden air tends to escape through a space between the misaligned surfaces. Secondly, since the moisture laden air is supplied from a pressurized air source, it is necessary to keep the compressor operating even if the system is left unattended for a long period of time, which is undesirable from the power savings standpoint.
One approach to these problems would be to submerge the nozzle into water when the system is not in operation. However, the water tends to introduce into the nozzle and the ink in the nozzle would be excessively diluted with the result that upon resumption of the writing operation the initial printing is unsatisfactory.